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Double bonds

  1. Nov 18, 2004 #1
    Why in organic chemistry do double bonds not allow rotation of bonds, whilst single bonds do?

    Thanks. :smile:
     
  2. jcsd
  3. Nov 18, 2004 #2
    Picture bonds in three dimensions to explain things like this. A single bond will be created by the area of intersection between the axial s-orbitals, which point toward eachother. A double bond, on the other hand, is created by the overlap of parallel vertically-oriented p-orbitals. If you rotate the atoms relative to eachother, the s-orbital continue to overlap as before, while the p-orbitals take on what you could call a staggered position (when viewed from the side), so overlap does not exist.
     
  4. Nov 19, 2004 #3
    Ok, so they rotate polarised light - my questions now are:

    a) Why? How do they cause it to rotate?

    b) Why do they turn it the opposite ways?

    Thanks. :smile:
     
  5. Nov 19, 2004 #4
    Sorry - put that on the wrong one of my posts! :uhh:

    The question on THIS post should be "So, what happens if you try to rotate a double bond? Does it resist rotation or is it still possible?"

    Thanks. :smile:
     
  6. Nov 19, 2004 #5

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    Rotating the double bond would eliminate the overlap between the p-orbitals, as Sirus described. You would need enough energy to break the C-C pi bond, which isn't common at normal temperatures. It is possible, however, at high temperatures.
     
  7. Nov 20, 2004 #6
    Why is it though that the p orbital electrons are the ones which become delocalised and form the pi bonds after the other electrons in the outer shell have formed sigma bonds with 2 hydrogens and the other carbon? After, surely we would expect the electron with the highest energy to react first ie the p orbital - not the s orbitals?

    Thanks. :smile:
     
  8. Nov 21, 2004 #7

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    The electrons are trying to adopt the lowest possible energetic configuration though, right? S orbitals are lower energy than p orbitals, so the lower energy molecular orbitals will have a larger contribution from the s atomic orbitals. In the case of an alkene the last p orbital is what is "left over."
     
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